Same reasoning as in 1cd251ed78

These validation tasks should only run when Prometheus is enabled.

It seems like it doesn't cause any issues for any of these services.

The quotes around "host" for both `--pid` and `--net` were
causing trouble for me:

> docker: --pid: invalid PID mode.

and:

> docker: Error response from daemon: network "host" not found.

I've also changed the `-v` call to `--mount` for consistency with the
rest of the playbook.

By running it in a more privileged container with access to the host network stack and such

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far